1. Technical Field
The present invention relates to a liquid crystal display device and a process for manufacturing the same. More particularly, the present invention relates to a liquid crystal display device employing transverse electric field type, in which a layered structure deposited on the substrate thereof is simplified and a channel region is securely insulated and moisture resistant, and also to a process for manufacturing the same.
2. Related Art
A widely used type of liquid crystal display devices is a vertical electric field type. Typically the liquid crystal display device of the vertically electric field type is configured to have a pair of transparent substrates on each of which an electrode unit and the like are formed and a liquid crystal layer sandwiched between the pair of transparent substrates. In operation, voltages are applied to electrodes on both substrates so as to reorient molecules of the liquid crystal for displaying various types of information. Such liquid crystal display device employing the vertical electric field type generally uses twisted nematic (TN) type, however such device undesirably exhibits a narrow viewing angle. Accordingly, the liquid crystal display device implementing other improved vertical electric field type has been developed. The improved vertical electric field type is typified by a vertical alignment (VA) type and a multidomain vertical alignment (MVA) type.
On the other hand, there have been also known liquid crystal display devices which employ an in-plane switching (IPS) type or a fringe field switching (FFS) type with a pair of electrodes formed on only one substrate, one electrode unit serving as pixel electrodes and the other electrode unit serving as a common electrode, unlike above mentioned liquid crystal display device of the vertical electric field type.
In the IPS-type liquid crystal display device, the pair of electrode units are formed on a common layer and an electric field is applied to a liquid crystal substantially in parallel with a substrate so as to reorient molecules of the liquid crystal substantially in parallel with the substrate. Therefore, the IPS-type liquid crystal display device is also called “liquid crystal display device of transverse electric field type” and advantageously exhibits a much wider viewing angle comparing to the foregoing liquid crystal display device of the vertical electric field type. However, in the IPS-type liquid crystal display device, the pair of electrode units for applying the electric field to the liquid crystal are formed on the common layer. Such structure hampers activation of liquid crystal molecules distributed in a region above and away from the pixel electrodes, so that transmittance and the like are disadvantageously decreased.
To obviate such disadvantage of IPS-type liquid crystal display device, there has been developed a so-called diagonal electric field type, or a FFS type (see JP-A-2001-235763 and JP-A-2002-182230). This FFS-type liquid crystal display device has a pixel electrode unit and a common electrode unit for applying an electric field to a liquid crystal layer thereof, each electrode unit being formed on different layers with an insulating layer (hereinafter referred to as “electrodes-insulation film”) sandwiched therebetween.
The FFS-type liquid crystal display device exhibits a wider viewing angle and higher contrasts than that of the IPS-type liquid crystal display device. Further, the FFS-type liquid crystal display device can exhibit a brighter display due to its higher transmittance while being capable of an operation at a low voltage. Also, in the FFS-type liquid crystal display device, an area in which the pixel electrode unit and the common electrode unit overlap is wider than that of the IPS-type liquid crystal display device, when viewed in plan. Such structure subsidiary generates higher supplemental capacity, whereby a line for supplying supplemental capacity does not need to be prepared separately, so that the FFS-type liquid crystal display device can exhibit higher aperture ratio.
However, the FFS-type liquid crystal display device disclosed in JP-A-2001-235763 and JP-A-2002-182230 causes electric potential difference between signal lines and the pixel electrode unit, that results in an irregular alignment of the liquid crystal molecules in vicinities of the signal lines. Therefore regions of the liquid crystal located in the vicinities of the signal lines are not substantially used for display, so that the aperture ratio disadvantageously decreases. Moreover, a capacitive coupling occurs between the signal line and the pixel electrode unit, leading to, for example, a crosstalk which impairs displaying quality. Further more, layers overlying the substrate are plural in number, so unevenness, or bumps are generated on the pixel electrode unit and a surface of the substrate, and that causes inequalities of a cell gap formed thereon. In order to address these problems, it has been proposed to employ a film as an intermediate layer used in the VA-type or MVA type liquid crystal display device and to arrange the pixel electrode unit and the common electrode unit on this film, to thereby decrease adverse effects of the electric potential of the signal line while minimizing the size of the bumps on the substrate surface (see JP-A-2001-83540 and JP-A-2007-226175).
In the liquid crystal display device disclosed in JP-A-2001-83540 and JP-A-2007-226175, the intermediate film is formed to obtain the high aperture ratio. The intermediate film is formed in the same manner as that of the liquid crystal display device of the transverse electric field type disclosed in JP-A-2001-235763 and JP-A-2002-182230. More particularly, the intermediate film is deposited on a surface of a passivation film deposited to overlie an entire displaying area including channel regions of switching devices, the passivation film being made from resin such as acrylic resin.
Although the aperture ratio is improved in the liquid crystal display device disclosed in JP-A-2001-83540 and JP-A-2007-226175, manufacturing this type of liquid crystal display device requires depositing many film structures on one of the transparent substrate thereof, so that many manufacturing process steps are unavoidably needed, resulting in high costs.
Incidentally, the switching device used in such liquid crystal display device has a characteristic variability under exposure to outside moisture, oxygen and so forth. In view of the foregoing, the liquid crystal display device of the transverse electric field type disclosed in JP-A-2001-235763 and JP-A-2002-182230 employs a intermediate film formed from photosensitive resin or the like and deposited overlying a passivation film which is formed to secure insulation and moisture resistant properties of channel regions of switching devices.
Both passivation film and intermediate film are deposited also in the liquid crystal display device of the transverse electric field type disclosed in JP-A-2001-83540 and JP-A-2007-226175, as is the case with the liquid crystal display device of JP-A-2001-235763 and JP-A-2002-182230. However, the channel region and the like of the switching device are covered not only by an insulation film of two layers, i.e. the passivation film and the intermediate film, but also by an electrodes-insulation film formed between pixel electrode unit and common electrode unit in the liquid crystal display device of the transverse electric field type disclosed in JP-A-2001-83540 and JP-A-2007-226175. That is, the cannel regions of the switching devices of the liquid crystal display device of the transverse electric field type are covered with three-layered insulation film formed of the passivation film, the intermediate film, and the electrodes-insulation film.
Therefore, the number of the manufacturing process steps is undesirably increased from that of the typical liquid crystal display device of the transverse electric field type, in the liquid crystal display device of the transverse electric field type disclosed in JP-A-2001-83540 and JP-A-2007-226175.
Inventors of the application have kept persistent diligence in investigation of the structure of the liquid crystal display device of the transverse electric field type, discovered that purpose for forming the passivation film, or securing insulation and water resistant properties, is attained by the electrodes-insulation film because the electrodes-insulation film and the passivation film are formed of the same material such as silicon nitride, and accomplished the present invention.